(1) Field of the Invention
This invention is related to MEMS devices, and more particularly, to methods of packaging MEMS devices in a vacuum cavity.
(2) Description of the Related Art
Micro-electro-mechanical systems (MEMS), the smallest functional machines that can be manufactured currently, are made up of components ranging from a few micrometers to several millimeters in size. MEMS, a rapidly growing semiconductor field, has many important practical and potential commercial applications. There are a range of commercially available MEMS products including gyroscopes, pressure sensors, fluid regulators, optical switches, displays, mass data storage, biological sensors and chemical controllers, and so on.
A scanning mirror type of MEMS device usually relies on the mechanical oscillation of a suspended microstructure via one or pairs of torsion beams. The structure is electrostatically deflected by energizing at least one actuation electrode in interdigital comb or parallel plate configuration. In order to minimize any air damping or random disturbance during operation, such optical MEMS devices are preferred to be vacuum packaged. Vacuum packaging also provides a benefit of minimizing contamination adsorption.
Device level vacuum packaging involves a complicated package design and sealing process. Commonly, the vacuum package using a lid seal method requires a through hole on the lid, a collapsible sealing gap, or expensive machines having the capability of manipulating assembly parts in a vacuum so the package cavities can be evacuated through the venting hole or the sealing gap or when the parts are not in an intimate contact position. The vacuum sealing is closed by a final solder reflow sealing process. Furthermore, before vacuum seal, the package materials usually require extended hours of bake out at an elevated temperature in order to prevent significant outgassing.
<<Wafer-level hermetic cavity packaging>> by George A. Riley, Advanced Packaging Magazine, May 2004, discusses various types of packaging including vacuum packaging for MEMS devices. U.S. Pat. No. 6,297,072 to Tilmans et al discloses a vacuum packaging method for a MEMS device comprising making an indent in a solder ring, forming the vacuum cavity, then reflowing the solder to close the indent and seal the package.
A number of patents discuss MEMS packaging. U.S. Pat. No. 6,852,926 to Ma et al describes a seal ring contacting a bond ring to form a hermetic seal around a MEMS device in a cavity that may contain a vacuum. U.S. Pat. No. 7,491,567 to DCamp et al discloses a solder preform between the MEMS die and the seal ring. The preform is melted and then the package is sealed by soldering. U.S. Pat. No. 6,844,606 to Logsdon et al teaches first reflowing solder to attach a chip to a carrier, then using a solder alloy as a seal ring having a lower reflow temperature than the first solder. U.S. Pat. No. 7,517,712 to Stark shows a solder preform heated to form a solder joint. U.S. Pat. No. 7,298,030 to McWilliams et al describes a sealed chip package using a solder preform. U.S. Pat. No. 7,259,449 to Floyd teaches forming a metal seal, such as solder.